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Pterosaurs (, from the Greek
πτερόσαυρος, pterosauros, meaning "winged lizard", often referred
to as pterodactyls, from the Greek πτεροδάκτυλος, pterodaktulos,
meaning "winged finger" /ˌtɛrəˈdæktɨl/)
were flying reptiles of
the clade or order
Pterosauria. They existed from the late Triassic to the
end of the CretaceousPeriod
(228 to 65 million
years ago). Pterosaurs were the first vertebrates to evolve powered
flight. Their wings were formed by a membrane of skin, muscle, and
other tissues
stretching from the thorax to a dramatically
lengthened fourth finger. Earlier species had long, fully-toothed
jaws and long tails, while later forms had a highly reduced tail,
and some lacked teeth. Pterosaurs spanned a wide range of adult
sizes, from the very small Nemicolopterus
to the largest known flying creatures of all time, including
Quetzalcoatlus
and Hatzegopteryx.

Pterosaurs are sometimes referred to in the
popular media as dinosaurs, but this is
incorrect. The term "dinosaur" is properly restricted to a certain
group of terrestrialreptiles with a unique
upright stance (superorder Dinosauria), and therefore excludes the
pterosaurs, as well as the various groups of extinct aquatic
reptiles, such as ichthyosaurs, plesiosaurs, and mosasaurs.

History of discovery

The first pterosaur fossil was described by the
Italian
naturalist Cosimo
Collini in 1784. Collini misinterpreted his specimen as a
sea-going creature that used its long front limbs as paddles. A few
scientists continued to support the aquatic interpretation even
until 1830, when the German zoologist Johann
Georg Wagler suggested that Pterodactylus used its wings as
flippers. Georges
Cuvier first suggested that pterosaurs were flying creatures in
1801, and coined the name "Ptero-dactyle" 1809 for a specimen
recovered in Germany; however, due to the standardization of
scientific names, the official name for this species became
Pterodactylus,
though the name "pterodactyl" continued to be popularly applied to
all members of this first specimen's order.

Wings

Pterosaur wings were formed by membranes of skin and
other tissues, strengthened by various types of closely spaced
fibers called actinofibrillae. The membranes attached to the
extremely long fourth finger of each arm and extended along the sides of
the body. A bone unique to pterosaurs, known as the pteroid,
connected to the wrist and helped to support a membrane (the
propatagium) between the wrist and shoulder. It has been argued
that the pteroid might have been able to swing forward to extend
this membrane,, although this is strongly contested by other
researchers. In some later pterosaurs, the backbone over the
shoulders fused into a structure known as a notarium, which served to
stiffen the torso during flight, and provide a stable support for
the scapula (shoulder
blade).

There has been considerable argument among
paleontologists about whether the wings attached to the hindlimbs
as well. Fossils of the rhamphorhynchoid Sordes, the anurognathidJeholopterus,
and a pterodactyloid from the Santana
Formation seem to demonstrate that the wing membrane did attach
to the hindlimbs, at least in some species. However, modern
bats and flying
squirrels show considerable variation in the extent of their
wing membranes and it is possible that, like these groups,
different species of pterosaur had different wing designs. Indeed,
analysis of pterosaur limb proportions shows that there was
considerable variation, possibly reflecting a variety of
wing-plans. Many if not all pterosaurs also had webbed feet, and
although these have been considered to be evidence of swimming,
they may have had an aerodynamic function. However, a large number
of pterosaur trackways
are now known, with a distinctive four-toed hind foot and
three-toed front foot; these are the unmistakable prints of
pterosaurs walking on all fours.

Unlike most vertebrates, which walk on their toes
with ankles held off the ground (digitigrade), fossil
footprints show that pterosaurs stood with the entire foot in
contact with the ground (plantigrade), in a manner
similar to humans and
bears. Footprints from
azhdarchids
show that at least some pterosaurs walked with an erect, rather
than sprawling, posture.

Though traditionally depicted as ungainly and
awkward when on the ground, the anatomy of at least some pterosaurs
(particularly pterodactyloids) suggests that they were competent
walkers and runners. The forelimb bones of azhdarchids
and ornithocheirids
were unusually long compared to other pterosaurs, and in
azhdarchids, the bones of the arm and hand (metacarpals) were
particularly elongated, and azhdarchid front limbs as a whole were
proportioned similarly to fast-running ungulate mammals. Their hind
limbs, on the other hand, were not built for speed, but they were
long compared with most pterosaurs, and allowed for a long stride
length. While azhdarchid pterosaurs probably could not run, they
would have been relatively fast and energy efficient.

Reproduction

Very little is known about pterosaur
reproduction. A single pterosaur egg has been found in the quarries
of Liaoning, the same place that yielded the famous 'feathered'
dinosaurs. The egg was squashed flat with no signs of cracking, so
evidently the eggs had leathery shells, as in modern lizards. The
embryo's wing membranes were well developed, suggesting pterosaurs
were ready to fly soon after birth. This is corroborated by very
young animals found in the Solnhofen limestone beds, where they
presumably flew to the middle of a lagoon, fell in and drowned. It
is not known whether pterosaurs practised parental care, but their
comparatively early flight capabilities suggest the young were not
completely dependent on parents as most birds are.

A study of pterosaur eggshell structure and
chemistry published in 2007 indicated that it is likely pterosaurs
buried their eggs, like modern crocodile and turtles. Egg-burying would have
been beneficial to the early evolution of pterosaurs, as it allows
for more weight-reducing adaptations, but this method of
reproduction also would have put limits on the variety of
environments pterosaurs could live in, and may have disadvantaged
them when they began to face ecological competition from birds. The alternative would be for
the mother to retain the egg within the body until just prior to
hatching, as some lizards do, but archosaurs are incapable
of.

Evolution and extinction

Origins

Because pterosaur anatomy has been so heavily
modified for flight, and immediate "missing link" predecessors have
not so far been described, the ancestry of pterosaurs is not well
understood. Several hypotheses have been advanced, with the most
common in recent years being links to ornithodirans like Scleromochlus,
an ancestry among the archosauriforms like
Euparkeria (a
more traditional view), or related to prolacertiformes like
Sharovipteryx.
At least one pterosaur specialist, David Unwin, finds none of these
options convincing for various anatomical reasons.

Phylogeny and classification

Classification
of pterosaurs has historically been difficult, because there were
many gaps in the fossil
record. Many new discoveries are now filling in these gaps and
giving us a better picture of the evolution of pterosaurs.
Traditionally, they are organized into two suborders:

Rhamphorhynchoidea
(Plieninger, 1901): A group of early, basal ("primitive")
pterosaurs, many of which had long tails and short metacarpal bones in the wing.
They were small, and their fingers were still adapted to climbing .
They appeared in the Late Triassic period, and lasted until the
late Jurassic. Rhamphorhynchoidea is a paraphyletic group (since
the pterodactyloids evolved directly from them and not from a
common ancestor), so with the increasing use of cladistics it has fallen out
of favor in most technical literature.

The precise relationships between pterosaurs is
still unsettled. However, several newer studies are beginning to
make things clearer. Cladogram
simplified after Unwin.

Extinction

Competition with early avian dinosaur species may have
resulted in the extinction of many of the
pterosaurs. By the end of the Cretaceous, only large species of
pterosaurs are known. The smaller species seem to have become
extinct, their niche filled by birds, though a lack of small
pterosaurs in the fossil record could also be a result of poor
preservation due to the fragility of their skeletons. At the end of
the Cretaceous period, the great extinction which wiped out all
non-avian dinosaurs plus most avian dinosaurs as well, and many
other animals, seemed to also take the pterosaurs. Alternatively,
most pterosaurs may have been specialised for an ocean-going
lifestyle. Consequently, when the
K-T mass-extinction severely affected marine life that most
pterosaurs fed on, they went extinct.

Pterodaustro
was a Cretaceous pterosaur from South
America with a wingspan around 1.33 metres and with over 500
tall, narrow teeth, which
were presumably used in filter-feeding, much like modern flamingos. Also like flamingos,
this pterosaur's diet may have resulted in the animal having a
pink hue. It was South
America's first pterosaur find.

Quetzalcoatlus
had a wingspan of 10-11 metres (33-36 feet), and was among the
largest flying animals ever. It lived during the late Cretaceous
period.

Rhamphorhynchus
was a Jurassic pterosaur with a vane at the end of its tail, which
may have acted to stabilise the tail in flight.

Pterosaurs in popular culture

Pterosaurs are a staple of popular culture. While
the generic term "pterodactyl" is often used to describe these
creatures, the animal depicted is frequently a Pteranodon or
other specific species of pterosaur, or a fictionalized hybrid of
several species. Many children's toys and cartoons feature
"pterodactyls" with Pteranodon-like crests and long, Rhamphorhynchus-like
tails and teeth, a combination that never existed in nature.
However, at least one type of pterosaur did have at least the
Pteranodon-like crest and teeth--the Ludodactylus,
a name that means "toy finger" for its resemblance to old,
inaccurate children's toys. Notable examples of older fictional
works featuring pterosaurs include Arthur
Conan Doyle's book
The Lost World and the 1933 film King
Kong.

Living pterosaur hoax

It was reported in an article in The
Illustrated London News (February 9, 1856, page 166) that, in 1856,
workmen laboring in a tunnel for a railway line, between Saint-Dizier
and Nancy, in
France, were
cutting through Jurassic limestone when a large creature stumbled
out from inside it. It fluttered its wings, made a croaking noise
and dropped dead. According to the workers, the creature had a
wingspan, four legs joined by a membrane, black leathery skin,
talons for feet and a toothed mouth. A local student of
paleontology identified the animal as a pterodactyl. The report had
the animal turn to dust, as soon as it had died.

This incredible hoax was stimulated in part by
contemporary Franco-Prussian palaeontological rivalry. The Solnhofen
limestone from Bavaria (in which
Archaeopteryx
would later be discovered) was producing many prized fossils, each
of which was proudly announced by German paleontologists. The
tunnel in question was through limestone of similar age to the
Solnhofen Limestone, so it presented an opportunity for a shocking
story by the French.